Beiträge zur Chemie der Oxidhalogenide von Molybdän und Wolfram. V. Molybdänoxidtetrachlorid und Molybdänoxidtrichlorid

The formation enthalpies were ascertained from the solution enthalpies in 2 n NaOH resp. 2 n NaOH + 1% H₂O₂. The results of equilibrium measurements in the systems give the formation enthalpies and standard entropies: . The value of the standard entropy of the gaseous MoOCl₄ was estimated to be 91 (±3) cl. From the enthalpies and entropies of sublimation the values were obtained.     

Effects of olefins on the thermal decomposition of propane part V. Effect of butene-2-Cis

The thermal decomposition of butene-2-cis at low conversion and its effect on the pyrolysis of propane have been studied in the temperature range 779-812 K. It was established that 2-butene decomposes in a long-chain process, with the chain cycle (Besides the radical path, the molecular reaction can also play a role in the formation of the products.) The thermal decomposition of propane is considerably inhibited by 2-butene, which can be explained by the fact that the less reactive radicals formed in the reactions between the olefin and the chain-carrying radicals regenerate the chain cycle more slowly than the original radicals in the above chain cycle or in the reactions The reactions of the 2-propyl radical are further initiation steps. The ratios of the rate coefficients of the elementary steps of the decomposition (Table III) have been determined via the ratios of the products. Estimation of the radical concentrations indicated that only the methyl, 2-propyl and methylallyl radicals are of importance in the chain termination. On the basis of the inhibition-influenced curves, the role of the bimolecular initiation steps. could be clarified in the presence of 2-butene.     

Flash photolysis of biacetyl in gas phase. Rate constants of reactions between methyl and acetyl radicals

The flash photolysis of biacetyl produces CO, C₂H₆, and CH₃COCH₃ as main products, and in small amounts CO₂, C₂H₄, and CH₃CHO. The rate constants of reactions (2) and (3) of thermally equilibrated radicals were calculated from the amounts of products: .     

The kinetics of free radical chain reactions in solutions of trichlorofluorethylene in cyclohexane

The kinetics of the gamma-radiation-induced free radical chain reaction in solutions of C₂Cl₃F in cyclohexane (RH) was investigated over a temperature range of 87.5–200°C. The following rate constants and rate constant ratios were determined for the reactions: In competitive experiments in ternary solutions of C₂Cl₄ and C₂Cl₃F in cyclohexane the rate constant ratio k_2c/k_2a was determined By comparing with previous data for the addition of cyclohexyl radicals to other chloroethylenes it is shown that in certain cases the trends in activation energies for cyclohexyl radical addition can be correlated with the C? Cl bond dissociation energies in the adduct radicals.     

Effects of Olefins on the Thermal Decomposition of Propane Part IV. Influence of Propylene

On the basis of the thermal decomposition of mixtures of propylene and propane with molar ratios of 0.0–0.33 in the temperature range 779–812K, the influencing functions describing the inhibition by propylene of the decomposition of propane were determined. The rate-reducing effect is explained mainly by the reactions (in which ^.R = ^.H, ^.CH₃ and 2-?₃H₇) and also by the addition reactions It was established that the bulk of the allyl radicals formed participate in the chain step, but, due to their lower reactivity, they restore the decomposition chain more slowly than the original radicals do. From the characteristic change in the ratio υ/υ, the rate ratios of hydrogenabstraction reaction by radicals from propylene and propane could be determined. In these reactions there was no significant difference between the selectivities of the radicals. For an interpretation of the changes, the decomposition mechanism must be completed with the reaction Evaluation of the influencing curves revealed that the initiation reactions must be taken into account. By parameter estimation we have determined the rate ratios characterizing the above initiation reactions, the unimolecular decomposition of propane, hydrogen abstraction by radicals from propane and propylene, intermolecular isomerization of the 2-propyl radical via propane and propylene, and abstraction of propane hydrogens by the ethyl and methyl radicals; these are given in Tables II.     

Kinetics of the reactions of cyclopropane derivatives. III. Gas-phase unimolecular isomerization of cyclopropyl cyanide to the cyanopropenes

Cyclopropyl cyanide isomerizes in the gas phase at 660°–760°K and 2–89 torr to give mainly cis- and trans-crotonitrile and allyl cyanide, with traces of methacrylonitrile. The reactions are first order, homogeneous, and unaffected by the presence of radical-chain inhibitors. The rate constants are given by Overall: cis-Crotonitrile: trans-Crotonitrile: Allyl cyanide: where the error limits are standard deviations. On the basis of a biradical mechanism, it is deduced that the ? CH? CN radical center is resonance stabilized by ca. 30 kJ mole^?1. Approximate equilibrium data are given for interconversion of the 1- and 3-cyanopropenes.     

The kinetics of thermal dimerization of hexatriene

The thermal isomerization of cis-hexatriene (cHT) to cyclohexadiene (CHD) and the dimerization of CHD and trans-hexatriene (tHT) in the liquid phase in the temperature range 380 K-473 K are reported. The rate coefficients are: for the cHT to CHD isomerization for tHT dimerizationlog and for CHD dimerization; endo form exo form © 1993 John Wiley & Sons, Inc.     

The kinetics of the thermal gas-phase reaction Br2 + C6F5I → BrI + C6F5Br. An attempt to determine the bond dissociation energy D(C6F5-I)

The kinetics of the thermal bromination reaction have been studied in the range of 261°–391°C. The observed rate law is compatible with initiation by the step for which we obtain where Θ = 2.303RT cal/mol. Using the above value of E₆, we have This result disagrees with values of D(C₆F₅-I) obtained in other ways and we conclude that reaction (3) probably does not involve initiation by reaction (6). Instead, initiation may involve an addition of Br to the ring in C₆F₅I followed by decomposition of the adduct to give C₆F₅Br. If correct, this implies that the Arrhenius parameters above refer to the addition reaction rather than to reaction (6).     

Lösungsthermodynamik von FeCl2 in Mischschmelzen aus Alkalichloriden und LaCl3 bzw. CeCl3

Solution Thermodynamics of FeCl₂ in Molten Mixtures of Alkaline Chlorides and LaCl₃ or CeCl₃ Activity coefficients and the chemical excess potential of FeCl₂ dissolved in molten chloride mixtures were determined by EMF measurements with galvanic cells of the type in the concentration range from 0.01–5 mole-% at 720 and 820°C. An average cationic potential is defined and used to calculate a distance parameter () for the different solvent melt mixtures. may be estimated by equations of the type      

Thermal decomposition of azoisopropane. I. Primary decomposition processes

The thermal decomposition of azoisopropane (AIP) was studied by detailed product analysis in the temperature and pressure intervals 498–563 K and 0.67–5.33 kPa. Besides the predominant termination and hydrogen-abstraction reaction of the 2-propyl radical, the decomposition is characterized by a very short chain process. The following rate constants were determined from the measurements for the following reactions:      

Effects of alkenes on the thermal decomposition of propane part II. Influence of ethylene

Experiments with propane-ethylene mixtures in the temperature range 760–830 K resulted in refinement of the role of ethylene inhibition in the decomposition of propane. The source of the rate-reducing effect of ethylene is the reaction This replaces the decomposition chains more slowly by means of the reactions than H-atoms do by direct H-abstraction from propane. Analysis of the ratios of the product formation rates showed that the selectivity of the ethyl radical for the abstraction of hydrogen of different bond strengths from propane was practically the same as that of the H-atom. The ratio of the rate constants of hydrogen addition to ethylene and methyl-hydrogen abstraction from propane by the H-atom (3) was determined as was that of the decomposition and the similar H-abstraction of the ethyl radical Interpretation of the influence of ethylene required the completion of the mechanism with further initiation of the reaction besides termination via ethyl radicals.     

The kinetics of the reactions of i-C3F7 radicals with BR2 and HBr

The reactions have been studied competitively in the vapor phase over the range of 52–204°C. The i-C₃F₇ radicals were generated by means of the reaction It was found that where θ = 2.303RT J/mol. Absolute Arrhenius parameters are derived for the reactions where R = CF₃, C₂F₅, and i-C₃F₇.     

Kinetics of the thermal gas-phase isomerization of 1,2-epoxycyclohexane

The kinetics of the gas-phase decomposition of 1,2-epoxycyclohexane has been studied over the temperature range 680–740 K at pressures between 1.6 and 6 torr. Isomerization to cyclohexanone and 2-cyclohexen-1-ol accounts for ca. 97% of the primary reaction products and occurs by first-order, homogeneous, nonradical processes:      

Kinetics and the mechanism of the thermal reaction between trifluoromethylhypofluorite and hexafluoropropene

The kinetics of the thermal reaction between CF₃OF and C₃F₆ have been investigated between 20 and 75°C. It is a homogeneous chain reaction of moderate length where the main product is a mixture of the two isomers 1-C₃F₇OCF₃ (68%) and 2-C₃F₇OCF₃ (32%). Equimolecular amounts of CF₃OOF₃ and C₆F₁₄ are formed in much smaller quantities. Inert gases and the reaction products have no influence on the reaction, whereas only small amounts of oxygen change the course of reaction and larger amounts produce explosions. The rate of reaction can be represented by eq. (I): The following mechanism explains the experimental results: Reaction (5) can be replaced by reactions (5a) and (5b), without changing the result: Reaction (4) is possibly a two-step reaction: For ∣CF₃ = ∣C₃F₆∣, ν_20°C = 36.8, ν_50°C = 24.0, and ν_70°C = 14.2.     

Pyrolysis of tetrafluorohydrazine

The pyrolysis of tetrafluorohydrazine has been studied from 578 to 791°K. The stoichiometry has been established as Reaction rates have been measured and the effects of surface area, inert gas pressure, and nitric oxide have been examined. The rate-determining step proposed is and the general rate expression obtained for this is      

Photolysis of azomethane–propane mixtures. Arrhenius parameters for the hydrogen abstraction reactions of methyl with azomethane and with propane

Mixtures of up to 14% azomethane in propane have been photolyzed using mainly 366 nm radiation in the ranges of 323–453 K and 25–200 torr. Detailed measurements were made of the yields of nitrogen, methane, and ethane. Other products observed were isobutane, n-butane, ethene, and propene. A detailed mechanism is proposed and shown to account for the observed variation of product yields with experimental conditions. The quantum yield of the molecular process is found to be given by the temperature-independent equation The values of rate constants obtained are where the reactions are and it is assumed that the rate constant for the reaction is given by      

Kinetics,mechanism, and endo selectivity of Diels–Alder reactions of alkylmonosubstituted ethenes with cyclohexa-1,3-diene in the gas phase

The reactions where Y = CH₃ (M), C₂H₅ (E), i? C₃H₇ (I), and t? C₄H₉ (T) have been studied between 488 and 606 K. The pressures of CHD ranged from 16 to 124 torr and those of YE from 57 to 625 torr. These reactions are homogeneous and first order with respect to each reagent. The rate constants (in L/mol·s) are given by The Arrhenius parameters are used as a test for a biradical mechanism and to discuss the endo selectivity of the reactions.     

Rate coefficients of decomposition of azoethane and azoisopropane

The thermal decompositions of azoethane and azoisopropane were studied in a large excess of ethylene in the temperature interval of 523–623 K. It was demonstrated that under such conditions, the bulk of the alkyl radicals react with ethylene. Via measurements on the consumption of the azo compound and on the formation of gaseous nitrogen, it was possible to determine the rate coefficients of the initiation reactions: . The resulting data were as follows: For azoethane, . For azoisopropane,      

Time-resolved vibrational chemiluminescence: Rate constants for the reactions of F atoms with H2O and HCN,and for the relaxation of HF (v = 1) by H2O and HCN

Rate constants have been determined at (298 ± 4) K for the reactions: and the relaxation processes: Time-resolved HF(1,0) emission was observed following the photolysis of F₂ with pulses from an excimer laser operating on XeCl (λ = 308 nm). Analysis of the emission traces gave first-order constants for reaction and relaxation, and their dependence on [H₂O] and [HCN] yielded:      

Thermal stability of alcohols

3,3-Dimethylbutanol-2 (3,3-DMB-ol-2) and 2,3-dimethylbutanol-2 (2,3-DMB-ol-2) have been decomposed in comparative-rate single-pulse shock-tube experiments. The mechanisms of the decompositions are The rate expressions are They lead to D(iC₃H₇? H) – D((CH₃)₂(OH) C? H) = 8.3 kJ and D(C₂H₅? H) – D(CH₃(OH) CH? H) = 24.2 kJ. These data, in conjunction with reasonable assumptions, give and The rate expressions for the decomposition of 2,3-DMB-1 and 3,3-DMB-1 are and